The present invention relates to an image forming apparatus used as an electrophotographic copying machine.
A paper sheet is usually discharged from a copying machine main body onto a discharge-paper tray projecting from the copying machine main body. In recent years, however, to save space, an intra paper-discharge type of copying machine having the paper-discharge tray within the main body, has been in the mainstream.
Another type of the copying machine is known which has an automatic inverting unit for attaining two-sided copy. The automatic inverting unit is arranged within a main body in the copying machine in which a paper discharge tray projects therefrom, whereas it is attached to an outer side surface portion of the copying machine main body in the intra-discharge type copying machine.
When the two-sided copy is made, the paper sheet on the upper surface side of which an image is copied in an image transfer section, is fed into the automatic inverting unit. The paper sheet is then inverted by the automatic inverting unit and again fed into the image transfer section to transfer an image onto the rear surface side of the paper.
In some copying machines of the intra-discharge type, the paper sheets having an image on the upper surface side are discharged outside from a paper-discharge roller pair in a predetermined amount, and thereafter, taken into the copying machine and sent into the automatic inverting unit by rotating the discharge roller pair in a reverse direction, whereby a two-sided copy can be made.
The paper-discharge roller pair plays a role in corrugating the paper sheet when the paper sheet is discharged through the paper-discharge roller pair. As a result, the paper sheet discharged onto the paper-discharge section acquires structural strength and improved in alignment.
However, when the corrugated paper sheet is returned again to the copying machine main body and fed to the automatic inverting unit to make a rear-side copy, if the paper is corrugated strongly in order to improve the alignment properties of the discharged paper sheet, the paper sheet comes into collusion with a photosensitive drum, leading to a paper jam. Alternatively, the paper sheet is pressed against the photosensitive drum to produce wrinkle at the time of copying the rear surface thereof.
Therefore, in the conventionally-used machine, the paper sheet is weakly corrugated to prevent occurrence of the paper jam or wrinkle at the time copy is made on the rear surface of the paper sheet. However, when the paper sheet is weakly corrugated, there is a problem in that the alignment of the discharged paper sheet decreases.